To treat cancer, a surgery is generally performed and mostly accompanied with chemo-therapy and radio-therapy. However, these treatment methods have serious side effects of destroying normal tissues and the treatment effect of them varies with patients. To improve the above disadvantage of the conventional methods, a “targeting method” emerges as an alternative, which is to target a specific protein particularly expressed in cancer cells and thus to develop an anticancer agent by using an inhibitor or an antibody that binds to the target protein. This agent cannot bind to or invade into normal tissues but is able to interact specifically with cancer cells, making it a promising agent with reduced side effects.
One example of the “targeting treatment” is immuno therapy, which is a method to kill cancer cells, based on the innate immune system. Particularly, an antibody binding to a cancer specific protein is injected and then bound to the cancer cells to cause immune response, resulting in the death of the cancer cells. Therapeutic antibodies developed so far in USA are approximately 10, which have been used as a therapeutic agent for leukemia or breast cancer. However, a proper immuno-therapy for other cancers has not been developed, yet.
One of the representative method for the immuno-therapy is monoclonal antibody therapy. An antibody has a high selective binding capacity to its target antigen and thus exhibit anticancer effect via natural immune system. One of the constituents of an antibody, Fab domain, forms an antigen binding site and Fc domain responses to those cells involved in immune system to cause immune response. Cancer cells are directly affected by the binding of the Fab domain of an antibody to its target antigen. In the meantime, the bond between Fcγ receptor of an effector cell and Fc domain of an antibody can induce antibody-dependent cell-mediated cytotoxicity and induce cell lysis by mediating Fc-mediated complement reaction (Kabita M D, Madhav V, PNAS, 102:18; 6243-6244, 2005). Therefore, the monoclonal antibody therapy is in the limelight of the medical field owing to such expecting anti-cancer effect and less side effects.
An antibody is functioning to induce antibody-dependent cell-mediated cytotoxicity (ADCC), complement dependent cytotoxicity (CDC) and phagocytosis at the same time to inhibit cell surface receptors. In general, ADCC and CDC are major mechanisms of immune defense system.
ADCC is a kind of cytotoxicity randomly executed by those cells recognizing Fc receptor such as natural killer cells, leukocytes and macrophages. When a specific antibody is bound to a target protein of a target cell, the target cell becomes lysed by the action of effector cells. The effector cells for ADCC analysis in vitro are peripheral blood mononuclear cells (PBMC) and natural killer cells, and these cells can be obtained from the blood of volunteers.
CDC is induced by the action of a complement in serum, which also causes target cell lysis. The activation of a complement begins with the formation of a complex of C1q, the first element of complement system, and an antibody on a target cell. To analyze CDC, serum without heat-inactivation at 56° C. is used.
In the meantime, CD24 has been identified as a homologous protein of a murine heat-stable antigen (HAS; mouse CD24) in late 1970. HAS is a glycoprotein of a mouse, which is connected to cell membrane by fixed glycosylphosphatidylinositol (GPI) and composed of 31 amino acids in total. Among these amino acids, 16 amino acids are Ser, Thr and Asn residues that can be O-glycosylated and N-glycosylated (Kay R et al, J immunol 147:1412-1416, 1991). The potential O-glycosylation sites are mainly located in N-terminal and C-terminal of CD24, so it is expected that CD24 has a dumbbell like shape. Glycosylation of CD24 depends on cell types and the glycosylated molecule has a wide range of molecular weight of 35 kDa-70 kDa (G. Kristiansen et al. J Mol Histology 35:255-262, 2004). Mouse HAS is mostly expressed in hematopoietic cell subpopulation including pro-B lymphocytes but is also found in the brain and epithelial cells of the tissue under the developmental stage (Belvindrah R et al., J Neuroscience 22:3594-3607, 2002).
Human CD24 has a similar expression pattern to mouse HAS but unlike mouse HAS, human CD24 is not expressed in erythrocytes and thymocytes and only found in early stage B-lymphocytes (Kay R et al., J immunol 147:1412-1416, 1991). Therefore, CD24 has been used as an early stage B-cell marker. Later, it has been confirmed that CD24 could be used as a marker for epidermal cells of the kidney and the brain under the developmental stage. A CD24 knock-out mouse had no other functional defect but B-lymphocyte development (Nielsen P J et al., Blood 89:1245-1258. 1997; Shirasawa T. et al, Dev Dyn 198:1-13, 1993), indicating that CD24 is involved in the proliferation and maturation of pro-B-lymphocytes.
According to the recent immunohistochemical studies, CD24 over-expression is observed in various cancer cells. In particular, it was confirmed that once CD24 was found in cytoplasm, metastasis was accelerated. From the immunohistochemical tests, it was confirmed that CD24 over-expression was observed in ovarian cancer (83%), breast cancer (85%), small cell lung cancer (45%), prostatic cancer (48%), pancreatic cancer (72%), rectal cancer (84%), cholangiocarcinoma (51%) and bladder cancer (62%). Comparatively high CD24 expression in lethal cancers showing low survival rate such as ovarian cancer, breast cancer, small cell lung cancer and prostatic cancer suggests that CD24 can be used as a diagnostic marker for those cancers (Krisiansen G. et al., Am J Pathol, 161:1215-1221, 2002; Krisiansen G. et al., Clin cancer Res, 9:4906-4913, 2003; Krisiansen G. et al., Br J Cancer, 88:231-236, 2003; Kristiansen G. et al., Prostate, 58:182-192, 2004; Hocob J. et al., Pancreatology, 4:454-460, 2004; Samuel E D, BioMed Central, 3:3-15, 2004; Min-Cheng S. et al., Cancer letter, 1-6, 2005; Yoon-La C. et al. Archives of Pathology & Laboratory Medicine, in press). According to the data established by the inventors, CD24 over-expression is also observed in liver cancer, small intestine cancer, large intestine cancer and cervical cancer (LG database).
Therefore, the present inventors performed experiments by treating an antibody against pre-cancer marker CD24 into an ovarian cancer cell line. As a result, cancer was suppressed by CDC and ADCC and so the inventors further completed this invention by confirming that cancer can be inhibited by targeting CD24.